The present invention relates to fluid treatment apparatus, which can be used for example as an artificial kidney for haemodialysis and for ultrafiltration.
One form of apparatus of this type comprises a stack of plates each having ribs on their faces with two semi-permeable membranes between each pair of adjacent plates of the stack. The ribs on two adjacent plates and on which the membranes rest can be disposed at an angle to one another.
Apparatuses of this type are already known for the same use, but a new problem has now presented itself to those skilled in the art, because the recent membranes, which have been improved in respect of the performance characteristics relating to the exchanges of matter, that is to say the dialysis exchanges, have also been improved in respect of their ultrafiltration performance characteristics. Now, it is known that, for the treatment of patients by haemodialysis, it is necessary to limit the ultrafiltration during the sessions or, if appropriate, during periods of a session. In view of the fact that, for safety reasons, the blood which circulates between the membranes is at a higher pressure than the dialysis liquid circulating between the ribs and in contact with the plates, those skilled in the art have observed that, with the apparatuses which are provided with plates comprising conventional ribs, the pitch of which is generally less than 3 mm, it is impossible, with recent high-performance membranes, to limit the ultrafiltration in the desired manner, in particular in the region of 200 ml/hour/m.sup.2 (milliliters per hour and per m.sup.2 of useful membrane surface area).